Swing (originating from jazz) and notes inégales (from the baroque/classical era) are both names for a rhythmic technique wherein a set of notes (usually pairs of eighths or sixteenth notes), nominally equal in duration, are played unequally.
Swung duple notes, if notated explicitly, are usually termed a "triplet feel", and this is how most classically trained musicians learn it, as Western canon tends to strictly enforce regular divisions; however, study of historical and contemporary performance practice shows that swing and notes inégales exist on a continuum, from "straight" 1:1 up to a "hard" 3:1 ratio between long and short notes (think the Marche from Tchaikovsky's Nutcracker Suite) which is traditionally considered the limit of swing.
Different degrees of inequality give rise to different names, with the "hardness" or "heaviness" increasing as the inequality increases.
- "Light swing" denotes a small degree of inequality, from ~3:2 to as subtle as 7:5, and is more characteristic of classical/baroque practice. The golden ratio could be a good cutoff point.
- "Medium swing" denotes a greater degree of inequality, up to the vicinity of the triplet feel 2:1.
- "Hard swing" denotes a ratio noticeably higher than 2:1, up to the limit of 3:1. Silver ratio swing can be found here.
Swing can be applied to other note values as well. For example, Irish jigs make use of swing in triplets, where the first two notes of each triplet have medium-hard swing applied. The concept could potentially be extended to modify the ratio of all three notes to each other, for example, a subtly accelerating 6:5:4.